As portable wireless devices, such as cell phones, wireless personal digital assistants (PDAs), or the like, become increasingly ubiquitous, wireless communications protocols are continually evolving to expand supported feature sets. To maximize compatibility with as many different wireless communications systems as possible, modern portable wireless devices may be multi-mode devices, which are able to operate in one of multiple modes associated with different RF communications bands, different wireless communications protocols, or both.
Many portable wireless devices are capable of transmitting RF signals and include an RF transmitter, which has at least one RF amplifier, such as a power amplifier. To function properly, the RF amplifier may need a DC bias, which may be provided by amplifier bias circuitry. Characteristics associated with providing the DC bias may include a value of the DC bias, an output impedance associated with providing the DC bias to the RF amplifier, and a control loop bandwidth associated with a control loop. When switching between RF communications bands, between different wireless communications protocols, or both, any or all of the value of the DC bias, the output impedance associated with providing the DC bias to the RF amplifier, and the control loop bandwidth may need to be changed. For example, a required value of the DC bias may be based on an output power level from the RF amplifier, the output impedance associated with providing the DC bias to the RF amplifier may be based on linearity requirements of the RF amplifier, and the control loop bandwidth may be based on a modulation scheme used by the RF amplifier. Specifically, some wireless communications protocols have strict linearity requirements, which may require a low output impedance associated with the DC bias. Some wireless communications protocols, such as those that include amplitude modulation (AM), may require a wide control loop bandwidth. Thus, there is a need for amplifier bias circuitry that can support multiple RF communications bands, different wireless communications protocols, or both, and can vary any or all of the value of the DC bias, the output impedance associated with providing the DC bias, and the control loop bandwidth.